The three components of an aircraft's velocity are currently determined by comparing the stagnation pressure, measured by a pitot tube, with static pressure. Such devices are simple, reliable, relatively inexpensive and reasonably accurate. By their very nature, however, they must make their measurements in a region of space which is in close proximity to the aircraft and hence is disturbed by the presence of the aircraft itself. The pitot tubes, which protrude into the air mass, have the disadvantage of disturbing the very air mass which they are sensing. This problem becomes particularly acute near the speed of sound where the shock waves generated by the motion of the aircraft can significantly disturb the actual measurement. Pitot tubes and similar sensing elements increase drag on an air frame. In addition, prior art pneumatic systems have slow response times and are inaccurate at low velocities. As the accuracy of measurement of the parameter being measured decreases, the accuracy with which the aircraft can be controlled decreases. Accordingly, fuel and maintenance costs increase.
Another prior art technique for measuring gas velocities is one utilizing the laser Doppler principle and depends upon aerosol particles being constrained in the air. Aerosol particles are particles of water, dust or pollen that occur naturally in the atmosphere and range in size from several tens of microns down to almost molecular sizes. The smaller aerosol particles are actually entrained within the airstream and follow its motion exactly. Since these particles scatter light, it is possible to detect both their presence and motion. Thus by measuring the motion of the aerosol particles with respect to the aircraft, it is possible to deduce the velocity of the aircraft with respect to the air mass through which it is moving.
The laser Doppler velocimeter generates, in a volume remote from the aircraft, a fringe pattern resulting from constructive and destructive interference of two coherent beams of light. An aerosol particle traversing these fringes will alternately scatter light when it is in a region of constructive interference and not scatter light when it is in a region of destructive interference. A portion of the light is scattered back to the aircraft. Since the spacing between the fringes is known, the component of the aerosol particle's velocity normal to the fringe direction can be calculated by measuring the frequency of the back-scattered signal and combining this frequency with wavelength and beam separation parameters. As an example of a prior art device embodying the above principles, see U.S. Pat. No. 4,506,979 entitled "Compact Radiation Fringe Velocimeter for Measuring in Three Dimensions."
The laser Doppler velocimeter approach overcomes the disadvantages of the mechanical-pneumatic approach. It, however, does have the disadvantages of having a low sampling rate at high altitudes due to the absence of larger particles and consequently low data rates. It also requires coherent source and high energy to overcome background noise.
Accordingly, the present invention contemplates providing an improved non-invasive optical velocimeter which adds no drag to the aircraft's air frame, operates quickly and accurately, operates at low velocities, and has a high sampling rate and high data rates. In addition, the present invention is less susceptible to background noise.